The present invention relates to the diagnostic imaging arts. It finds particular application in conjunction with open MRI systems with a main magnetic field greater than 0.5 T and will be described with particular reference thereto. It will be appreciated, however, that the present invention is useful in conjunction with other systems containing more than one radio frequency coil, such as bore type MRI systems, spectroscopy systems, low field systems, and the like, and is not limited to the aforementioned application.
In magnetic resonance imaging, a uniform main magnetic field is created through an examination region in which a subject to be examined is disposed. With open magnetic resonance systems, the main magnetic field is typically vertical, perpendicular to the subject between upper and lower poles. A series of radio frequency (RF) pulses are applied to two RF coils, one adjacent each pole, to excite and manipulate magnetic resonance. Gradient magnetic fields are conventionally produced by gradient coils mounted between the RF coils and the poles to encode spatial position and other information in the excited resonance. The magnetic resonance signals are detected with the two RF coils or localized coils and processed to generate two or three dimensional image representations of a portion of the subject in the examination region.
Typically, the patient is placed in the examination region on his/her back close to the bottom pole assembly and further from the upper pole assembly to maximize openness in front of the patient. This causes uneven loading of the upper and lower RF coils during transmit, resulting in each coil having different RF power coupling to the patient. That is, the coils contribute unevenly to the imaging process. Patient geometry also contributes to different loading of the two RF coils. The anterior and posterior shapes of the body are not the same and are different at different positions along the length of the patient. The loading and RF power coupling experienced by the two coil assemblies due to this geometry variation are different.
With relatively low main fields of existing open systems, this phenomenon has not been a significant problem. However, as main fields increase, the uneven loading of the RF coils can become problematic in that the output images have non-uniform intensity such that they become unusable for diagnostic imaging.
In the higher fields of bore systems, a birdcage RF coil design is typically used, with the RF quadrature drive for the coil located 45 degrees to either side of the down position so that it is least affected by vertical patient location and shape. Generally though, the head to foot central axis of the patient is very close to the main axis of the bore, i.e. loading is relatively symmetric.
Conventionally, calibrated RF pulses are used to excite and manipulate the MR signal. That is, the excitation or tip angles, 180xc2x0 inversion angles, other spin system manipulation as well as which nuclei are resonated are precisely achieved with carefully calibrated RF pulses. In order to reorient the magnetization, the amplitude and phase of the RF envelope as a function of time is precisely controlled. The patient mass influences the loading of the RF coils which affects the reflected RF power and the forward RF power coupled into the coil. For each patient anatomy, the RF field is separately amplitude calibrated to achieve the proper tip angle. For a patient disposed symmetrically, this typically consists of exciting a transverse slab with a nominal 90xc2x0 pulse and adjusting the RF amplitude until a 90xc2x0 magnetization tip or flip angle is achieved. Depending on patient girth, the loading of the RF coils can differ greatly from patient to patient.
The present invention provides a new and improved method and apparatus that overcomes the above referenced problems and others.
In accordance with one aspect of the present invention, a magnetic resonance apparatus includes a vertical field main magnet system which generates a main temporally constant field through an examination region. An upper radio frequency coil and a lower radio frequency coil excite magnetic resonance in selected nuclei located in the examination region. An RF transmitter drives upper and lower RF coils that generate radio frequency magnetic fields that excite and manipulate magnetic resonance. The same coils may be used to receive resonance signals as well. An RF tip angle adjustment circuit is used to separately calibrate two RF coils to achieve the desired spin tip angles that are produced when driving the combined coils.
In accordance with a more limited aspect of the present invention, the tip angle analyzer contains a means by which it detects the tip angle caused by the RF coils. In order to adjust the tip angle for the desired effect, the RF tip angle analyzer changes the RF power directed to the transmitter coils.
In accordance with a more limited aspect of the present invention, the magnetic resonance apparatus transmits and receives magnetic resonance manipulation signals in quadrature.
In accordance with another aspect of the present invention, a magnetic resonance apparatus is given. A magnet assembly generates a main magnetic field through and examination region. First and second RF coil assembles are disposed opposite each other, adjacent the imaging region. An RF transmitter provides RF pulses to the coil assemblies. An RF monitor measures the RF power delivered by the coil assemblies separately. An RF tip angle calculator measures the tip of induced resonance in order to produce more desired tip angles.
According to a more limited aspect of the present invention, RF tip angle calculator retrieves adjustment data from a memory look up table.
According to another aspect of the present invention, a method of magnetic resonance is given. Two RF transmit coils are disposed adjacent an examination region. Magnetic resonance is selectively excited and tip angles of the resonance are measured and adjusted. The tip angles are adjusted by adjusting an RF pulse amplitude and a radio frequency.
According to another aspect of the present invention, a method of magnetic resonance is given. Two RF transmit coils are disposed adjacent an examination region. Magnetic resonance is selectively excited and tip angles of the resonance are measured and adjusted to produce a tip angle of a desired amount.
According to another aspect of the present invention, a method of magnetic resonance is given. Two RF coils are disposed opposite each other, adjacent an examination region. Total power and reflected power for each of the coils is measured. Power delivered into the examination region is calculated. The relative power to each coil is adjusted, thereby adjusting the tip angle in a selected region near a midplane.
One advantage of the present invention is that it provides RF transmit coils for open machines with higher main fields.
Another advantage of the present invention is that it permits off center placement of the subject, vertically and longitudinally.
Another advantage of the present invention is that it allows for images with better signal uniformity.
Another advantage of the present invention is that it provides better RF transmit fields for MRI.
Yet another advantage resides in a higher signal-to-noise ratio.
Still further benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments.